A carburetor is typically used to supply a fuel and air mixture to two stroke and four stroke internal combustion engines. For many applications where small two stroke engines are used, such as hand held power chainsaws, weed trimmers, leaf blowers, garden equipment and the like, a carburetor with both a diaphragm fuel pump and a diaphragm fuel metering system are often used.
A so-called diaphragm carburetor generally includes a body having a mixing passage with an air inlet opening, and a fuel and air mixture outlet opening downstream of the air inlet opening. The diaphragm carburetor also typically includes a throttle valve disposed in the fuel and air mixing passage downstream of the air inlet opening for controlling delivery of a primary fuel and air mixture to the engine. A typical diaphragm fuel metering system includes a metering valve, and a flexible diaphragm carried by the body and partially defining and separating a fuel chamber from the atmosphere. The metering valve and diaphragm provide fuel from the diaphragm fuel pump to the fuel chamber for delivery from the fuel chamber to the fuel and air mixing passage. A manually actuated purge pump is also carried by the body in fluid communication with the diaphragm fuel pump and fuel chamber and has a flexible bulb that is depressed to purge the diaphragm fuel pump and fuel chamber of any air and fuel vapor and stale liquid fuel. The flexible bulb is actuated to a depressed state, thereby causing liquid fuel and any fuel vapor and air within the bulb to be directed out of the carburetor through a downstream fuel line to a fuel tank. The flexible bulb is then returned to a non-depressed state, thereby drawing liquid fuel and any fuel vapor and air into the bulb through an upstream fuel line. The purging is generally repeated as necessary to ensure that fuel vapor, air and stale liquid fuel is purged from the fuel pump and fuel chamber.
Some diaphragm carburetors are also configured to supply an auxiliary fuel and air mixture in addition to the primary fuel and air mixture. For example, the auxiliary fuel and air mixture may be provided downstream of the throttle valve for improved engine starting and initial running of the engine just after starting. Typically, the auxiliary fuel and air mixture is provided from the fuel chamber through auxiliary fuel and air conduits and is controlled by manually actuated control valves and associated levers or knobs. In operation, a user first manually depresses the bulb of the purge valve to purge the fuel pump and chamber of vapor and air, then manually actuates the manual control valve to open the auxiliary fuel and air conduit, and starts the engine such as by pulling a manual start pull cord or energizing an electric starter. Thereafter, the user may have to manually actuate the manual control valve to close the auxiliary fuel and air conduit.
Thus, the manual control valve and associated devices add components, complexity, and cost to the carburetor, and require separate manual valve actuation steps by an operator.